1. Field of the Invention
The present invention relates to the field of communications. In digital communications each symbol transmitted over a time dispersive channel extends beyond the time interval used to represent that symbol. Distortion caused by the overlap of received symbols results in Inter-Symbol Interference (ISI), which may be reduced, by placing an equalizer in the path of the received signal. In particular, the invention relates to a method for changing the coefficients of an adaptive decision feedback equalizer, including its decision feedback and feed forward parts, to improve convergence.
2. Description of Related Art
The distortion caused by ISI may be reduced by passing the incoming signal through an adaptive decision feedback equalizer. Generally, the adaptation is performed according to some recursive method based on the minimization of means square error (MSE), for example, using a conventional least means square (LMS) or a recursive least squares (RLS) method. These conventional MSE based recursive adaptation equalizer methods exhibit convergence problems if the eigenvalues spread, e.g., the autocorrelation matrix condition number, representing a ratio between the largest to smallest eigenvalues, of the input signal is relatively large. Specifically, convergence speed depends on the spread of the eigenvalues of the input signal. Generally, the eigenvalue spread is relatively large due to the relatively high correlation of the input signal and depends on the length of the filter. A relatively large eigenvalue spread results in a relatively slow convergence requiring a very large number, for example, thousands or millions, of iterations that slowdown the adaptation process.
It is therefore desirable to develop a method that reduces the residual error and speeds up the convergence of any recursive mean square error based decision feedback equalizer using error prediction.
The present invention is directed to a method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus including an adaptive feed-forward equalizer and an adaptive decision feedback equalizer. An independent error predictor component is used for better convergence and then is eliminated by converting, using z-transformations, the adaptive feed forward equalizer and adaptive decision feedback equalizer to an equivalent feed forward equalizer and an equivalent decision feedback equalizer, respectively, in which the error predictor is embedded or incorporated therein. The smaller system with a reduced number of FFE-DFE coefficients has a faster convergence rate.
In one embodiment of the method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus in accordance with the present invention the adaptive feed-forward equalizer is transformed into an equivalent feed forward equalizer represented by a z-transformation
FFExe2x80x2(z)=FFE(z)[1xe2x88x92EP(ZN)]
where, FFExe2x80x2(z) is the equivalent feed-forward equalizer;
FFE(z) is an adaptive feed-forward equalizer;
EP(ZN) is an adaptive error predictor, wherein N is a sampling factor.
Similarly, the adaptive decision feedback equalizer is transformed into an equivalent decision feedback equalizer represented by a z-transformation
xe2x80x83DFExe2x80x2(z)=EP(z)+DFE(z)[1xe2x88x92EP(z)]
where, DFExe2x80x2(z) is the equivalent decision feedback equalizer;
DFE(z) is an adaptive decision feedback equalizer;
EP(Z) is the adaptive error predictor.
Another embodiment of the present invention is directed to a method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus, in which an initial tap length is set for each of the adaptive feed-forward equalizer and the adaptive decision feedback equalizer. During a first predetermined time period, the adaptive feed-forward equalizer and the adaptive decision feedback equalizer are adapted using a received distorted signal, until a desired error is obtained. An initial tap length is set for the error predictor. During a second predetermined time period, the adaptive feed-forward equalizer, the adaptive decision feedback equalizer, and the error predictor are simultaneously adapted using the received distorted signal. After that period, an equivalent feed-forward equalizer is determined using the z-transformation
FFExe2x80x2(z)=FFE(z)[1xe2x88x92EP(ZN)]
where, FFExe2x80x2(z) is the equivalent feed-forward equalizer;
FFE(z) is an adaptive feed-forward equalizer;
EP(ZN) is an adaptive error predictor, wherein N is a sampling factor.
Similarly an equivalent decision feedback equalizer is determined using the z-transformation
DFExe2x80x2(z)=EP(z)+DFE(z)[1xe2x88x92EP(z)]
where, DFExe2x80x2(z) is the equivalent decision feedback equalizer;
DFE(z) is an adaptive decision feedback equalizer;
EP(Z) is the adaptive error predictor.
Still another embodiment in accordance with the invention relates to a method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus, in which an initial tap length is set for each of the adaptive feed-forward equalizer and the adaptive decision feedback equalizer. During a first predetermined time period, the adaptive feed-forward equalizer and the adaptive decision feedback equalizer are adapted using a received distorted signal, until a desired error is obtained. An initial tap length is set for an error predictor. During a second predetermined time period, the error predictor is adapted using the received distorted signal. After that period, an equivalent feed-forward equalizer is determined using the z-transformation
FFExe2x80x2(z)=FFE(z)[1xe2x88x92EP(ZN)]
where, FFExe2x80x2(z) is the equivalent feed-forward equalizer;
FFE(z) is an adaptive feed-forward equalizer;
EP(ZN) is an adaptive error predictor, wherein N is a sampling factor.
Similarly an equivalent decision feedback equalizer is determined using the z-transformation
DFExe2x80x2(z)=EP(z)+DFE(z)[1xe2x88x92EP(z)]
where, DFExe2x80x2(z) is the equivalent decision feedback equalizer;
DFE(z) is an adaptive decision feedback equalizer;
EP(Z) is the adaptive error predictor.
The present invention is also directed to a hybrid decision feedback device for use with the methods described above. The device includes an equivalent feed-forward equalizer represented by a first z-transformation
FFExe2x80x2(z)=FFE(z)[1xe2x88x92EP(ZN)]
where, FFExe2x80x2(z) is the equivalent feed-forward equalizer;
FFE(z) is an adaptive feed-forward equalizer;
EP(ZN) is an adaptive error predictor, wherein N is a sampling factor.
The device also includes an equivalent decision feedback equalizer represented by a second z-transformation equation
DFExe2x80x2(z)=EP(z)+DFE(z)[1xe2x88x92EP(z)]
where, DFExe2x80x2(z) is the equivalent decision feedback equalizer;
DFE(z) is an adaptive decision feedback equalizer;
EP(Z) is the adaptive error predictor.